 NMR arises from the fact that certain atomic nuclei have a property called “ spin ”  “Spin” is caused by circulating nuclear charge and can be thought.

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Presentation on theme: " NMR arises from the fact that certain atomic nuclei have a property called “ spin ”  “Spin” is caused by circulating nuclear charge and can be thought."— Presentation transcript:

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 NMR arises from the fact that certain atomic nuclei have a property called “ spin ”  “Spin” is caused by circulating nuclear charge and can be thought of as bar magnet that adopts a preferred orientation in the presence of a static magnetic field Static Magnetic Field Generated by the NMR Spectrometer Nucleus of NMR Active atom  NMR spins adopt specific quantitized states; for spin ½ nuclei, the type of nuclei most commonly studied, there are two such states,  and .   corresponds to preferred low energy configuration, while  corresponding to the non-preferred high energy configuration. Nuclear Magnetic Resonance (NMR)

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 Energy can be applied by the NMR spectrometer in the form of an oscillating magnetic field at a precisely defined frequency (and hence energy) that induces transitions from  to .  Oscillating magnetic field is applied through the probe, which basically consists of a coil surrounding the NMR tube attached to a user controlled oscillating frequency source on the spectrometer console. Nuclear Magnetic Resonance (NMR)

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NMR Chemical Shifts  = physical constant for a given type of nucleus (ratio of magnetic moment and angular momentum) h = Planck ’ s constant B o = static magnetic field strength Prediction, based on the fact that all nuclei in the sample are placed in the same magnet, is that all nuclei of one type ( 1 H, 13 C, 15 N, 31 P etc) would have exactly the same NMR frequency

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Chemical Shielding Shielding arises from the various ways by which electrons “ shield ” the nuclear spin from the external magnetic field (B o ) Physical mechanism relates to induced circulation of electrons that oppose static magnetic field (Lentz ’ Law) Shielding (tensors) can be determined through ab initio calculations. This, however, is computationally expensive, and not routinely applied to large molecules, such as proteins.